Switching regulators are very commonly used in DC-DC conversion as they offer higher efficiency than linear regulators. In a basic form, a switching regulator, also referred to as a synchronous regulator, includes an inductor, a capacitor, a diode and a switch which switches the inductor alternately between charging and discharging states. These basic elements can be arranged to form, for example, a step-down (buck), a step-up (boost) and an inverting (buck-boost) regulator.
Control of the main switch or switches of a switching regulator may be achieved by techniques such as “voltage mode control” and “current-mode control.” In a basic constant frequency voltage-mode control, the duty cycle of the switch is controlled by comparing the signal at the output of an error amplifier with a predetermined duty-cycle-independent synchronous sawtooth signal. At the beginning of each cycle, the main switch is set on and kept on until the sawtooth ramp reaches the value of the error amplifier output signal. At that moment the switch is turned off and not turned on again until the beginning of the next clock cycle. In this way the output voltage of the regulator can be controlled to the required value.
For a basic constant frequency current-mode controlled buck converter, the main switch is connected to an input voltage and is closed at the beginning of a clock cycle. Closing the switch causes the current in an inductor connected between the switch and the output of the converter to rise. This current is monitored and compared against the output of an error amplifier. The error amplifier may be an internal error amplifier of a PWM controller of the converter. When the output voltage of the inductor current monitor exceeds the output voltage of the error amplifier the switch is turned off and not turned on again until the beginning of the next clock cycle. In this way the output voltage of the regulator may be controlled to the required value.
A simplified current-mode control circuit may include a comparator and an R-S flip-flop. The comparator can compare the sum of a signal proportional to the current through inductor and an optional artificial ramp signal to a threshold signal. The artificial ramp signal is a predetermined duty-cycle-independent synchronous sawtooth signal that may be added to the measured inductor current to address sub-harmonic oscillation. To ensure stability for all duty cycles up to 100%, the slope of the artificial ramp signal should be equivalent to at least half of the anticipated maximum inductor current down slope.
At the beginning of each switching cycle, the flip-flop can be set from an internal or external clock signal, which asserts the duty cycle signal high. The inductor current begins to rise and so does the artificial ramp. When the sum of the later two exceeds the control signal, the comparator output changes state which resets the flip-flop and terminates the duty cycle. The described control principle forces the inductor current to follow the control signal transforming the switching regulator's inductor into a controlled current source.